SAN FRANCISCO -- For 43 years, the Exploratorium in San Francisco has been distinguished by what was inside -- a museum designed to teach science through hundreds of quirky, hands-on exhibits, from pulleys to gears to microscopes -- part Willy Wonka's factory and part high school shop class.

But now the venerable institution that has hosted millions of Bay Area children on field trips since 1969 will be known as well for what is outside -- breathtaking waterfront views of San Francisco Bay -- as it reopens to the public next Wednesday in a gleaming new $300 million home.

At 330,000 square feet, the new Exploratorium, which opens April 17 along San Francisco's Embarcadero on Pier 15, is three times the size of the old one at the Palace of Fine Arts.

Visitors are mesmerized by their images reflected in a concave mirror at the Exploratorium's new $300 million home during a pre-opening tour led by staff scientist Paul Doherty, far left, on March 25, 2013. The facility in Pier 15 on San Francisco's waterfront opens April 17. (Karl Mondon/Staff)

And while that former home was windowless, aging and dusty, looking somewhat forlorn in comparison to the sleek Monterey Bay Aquarium and reborn California Academy of Sciences museum in Golden Gate Park, the new Exploratorium is a showplace on a 800-foot-long pier. Its huge glass windows open to the bay, and its expanded mission adds behavioral and environmental sciences to its teaching list alongside physics and biology.

"We have opened our windows," said Tom Rockwell, director of exhibits for the museum. "We have an outdoors."

Museum planners expect attendance to double at the new home, from 560,000 visitors last year to 1 million or more.

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With no new spaces constructed, parking could be a headache. The hope is that residents from the East Bay and Silicon Valley will take ferries, BART or Caltrain to the new site, just north of the Ferry Building. From opening day, April 17, through September, visitors will only be sold timed entry tickets to keep the crowds manageable.

About 150 of the roughly 600 exhibits are new, particularly those having to do with the natural environment.

"In the past we had a microscope center where we would look at micro-organisms," Rockwell said. "But now we can actually draw the water up from the bay and take stock of the changes on an hourly basis as new creatures come into the bay from the large cargo ships around the world."

Longtime Exploratorium visitors will recognize previous exhibits, like the "Vortex," a 10-foot high water tornado, or microscopes that show mice stem cells turning into beating heart cells.

There are a host of new exhibits, however, including a 22-foot-tall "Tinkerers' Clock," with pulleys, levers, gears and a gong, all run by windshield wiper motors; a 330-year-old Douglas fir tree that fell in a storm, sliced to show what tree rings reveal; and a new wind table where people can construct flying objects and test them in columns of moving air.

The building's marquee spot is a 6,000-square-foot glass observatory looking from the second floor across the bay. Sensors record water temperature, salinity, currents and other data in real time. And a "ship tracker" screen measures the transponder signals from large ships, broadcasting their positions and other information. Other exhibits show changes in the shape of the bay over centuries, 3-D topographic images, fog, fault lines and sediment cores.

"We need to remember the most powerful tool you have is a question," said Exploratorium CEO Dennis Bartels. "Our exhibits, when they at their best, make you ask: What just happened?"

The original Exploratorium was founded by physicist Frank Oppenheimer, who worked with his older brother, J. Robert Oppenheimer, in the 1940s on the Manhattan Project designing the nuclear bomb. After the war, Frank Oppenheimer was stripped of his professorship at the University of Minnesota during the McCarthy Era because of his involvement with the Communist Party in the 1930s. He bought a ranch in Pagosa Springs, Colo., and began teaching at a small high school.

He taught the rural students by taking them to a junkyard. They began winning first place in state science contests, and eventually the University of Colorado hired him. By 1969, Oppenheimer moved to the Bay Area and opened the Exploratorium, which he helped run until his death in 1985.

"Frank wanted the first Exploratorium to feel like a garage, like Los Alamos, where he and his brother worked," Bartels said. "We needed something that said, 'There is nothing precious in here.' Everything is to play with and enjoy. There's nothing here that can't be fixed. I think he'd be very happy with our new location."

Built in 1915, Pier 15 had been used most recently to repair transmissions on city buses. To bring it up to modern standards, work crews had to perform a massive overhaul. Starting in 2010, they ripped out and replaced more than 1,000 aging wooden pilings, and shored up both Pier 15 and the adjacent Pier 17, which will be used for storage, with huge steel supports. They also built a 300-foot-long seismic joint so the two piers can flex independently, making the whole structure capable of withstanding a magnitude 8.2 earthquake.

The old Exploratorium is being used now as a temporary private boys school. The city, which owns it, is taking offers from other museums and nonprofit organizations.

Through all the changes, the central philosophy behind the Exploratorium has been that kids learn best by doing things, rather than being told things.

"You can see the awe and the wow factor kicking in," said Gary Nakagiri, of El Cerrito, a retired middle and high school science teacher. "It gives kids a better sense of what science really is. And if we can keep them motivated, we can maintain their interest. Ultimately we hope those kids will become our engineers and scientists of the future."